1. Field of the Invention
The present invention relates to an optical transmission system applicable to a closed loop topology.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has already been developed to amplify signal light in a 1.55 μm band where the loss in a silica fiber is low. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 μm band or a 1.48 μm band, a gain band including a wavelength of 1.55 μm can be obtained.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
By using an optical amplifier as a linear repeater, the number of parts in the repeater can be greatly reduced as compared with the case of using a conventional regenerative repeater, thereby ensuring reliability and allowing a substantial cost reduction.
The introduction of WDM into a trunk line has been increasingly made in recent years, and as a network topology adopting WDM a ring configuration, or closed loop topology is generally used. However, the introduction of WDM into a medium-distance network (metropolitan area network) such as in urban areas delays in response to an increase in transmission speed in the trunk line, and an optical communication system by a single wavelength or an electrical communication system by a metal cable is yet mainly in use. Accordingly, the demand for the introduction of WDM into the metropolitan area network is increasing with the widespread use of the Internet and the trend toward broadband communication.
The metropolitan area network is usually configured by arranging a plurality of nodes along the closed loop topology. The reason for such a configuration is to provide a stable communication environment even when a failure such as a line break occurs.
In general, each node connected to the closed loop topology performs adding/dropping of an arbitrary one or more optical signals by utilizing the feature of WDM. To this end, each node employs an optical switch or the like in many cases, so that an optical amplifier is included in each node to compensate for loss in the optical switch. The optical amplifier is so designed as to support a difference between the wavelength of each optical signal passing through the node and the wavelength of each optical signal to be added/dropped and a change in the number of channels. Further, automatic level control (ALC) for maintaining a required output at a constant level is also necessary to tolerate a deviation in transmission line loss.
In performing ALC in each node, it is necessary to preliminarily grasp the number of channels of WDM signal light input to the optical amplifier. Therefore, a supervisory signal including information on the number of channels of the WDM signal light is transmitted from the upstream node to the downstream node. However, at initially starting (cold starting) the system, the number of add/drop channels has not been determined, so that the information on the number of channels of the WDM signal light is not determined and each node cannot be started easily.